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From: Pavel Begunkov <asml.silence@gmail.com>
To: Jens Axboe <axboe@kernel.dk>, Kanchan Joshi <joshiiitr@gmail.com>
Cc: Kanchan Joshi <joshi.k@samsung.com>,
	viro@zeniv.linux.org.uk, bcrl@kvack.org,
	Matthew Wilcox <willy@infradead.org>,
	Christoph Hellwig <hch@infradead.org>,
	Damien Le Moal <Damien.LeMoal@wdc.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-aio@kvack.org, io-uring@vger.kernel.org,
	linux-block@vger.kernel.org, linux-api@vger.kernel.org,
	SelvaKumar S <selvakuma.s1@samsung.com>,
	Nitesh Shetty <nj.shetty@samsung.com>,
	Javier Gonzalez <javier.gonz@samsung.com>
Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append
Date: Thu, 30 Jul 2020 19:26:10 +0300
Message-ID: <da4baa8c-76b0-7255-365c-d8b58e322fd0@gmail.com> (raw)
In-Reply-To: <80d27717-080a-1ced-50d5-a3a06cf06cd3@kernel.dk>

On 30/07/2020 19:13, Jens Axboe wrote:
> On 7/30/20 10:08 AM, Pavel Begunkov wrote:
>> On 27/07/2020 23:34, Jens Axboe wrote:
>>> On 7/27/20 1:16 PM, Kanchan Joshi wrote:
>>>> On Fri, Jul 24, 2020 at 10:00 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>
>>>>> On 7/24/20 9:49 AM, Kanchan Joshi wrote:
>>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>>> index 7809ab2..6510cf5 100644
>>>>>> --- a/fs/io_uring.c
>>>>>> +++ b/fs/io_uring.c
>>>>>> @@ -1284,8 +1301,15 @@ static void __io_cqring_fill_event(struct io_kiocb *req, long res, long cflags)
>>>>>>       cqe = io_get_cqring(ctx);
>>>>>>       if (likely(cqe)) {
>>>>>>               WRITE_ONCE(cqe->user_data, req->user_data);
>>>>>> -             WRITE_ONCE(cqe->res, res);
>>>>>> -             WRITE_ONCE(cqe->flags, cflags);
>>>>>> +             if (unlikely(req->flags & REQ_F_ZONE_APPEND)) {
>>>>>> +                     if (likely(res > 0))
>>>>>> +                             WRITE_ONCE(cqe->res64, req->rw.append_offset);
>>>>>> +                     else
>>>>>> +                             WRITE_ONCE(cqe->res64, res);
>>>>>> +             } else {
>>>>>> +                     WRITE_ONCE(cqe->res, res);
>>>>>> +                     WRITE_ONCE(cqe->flags, cflags);
>>>>>> +             }
>>>>>
>>>>> This would be nice to keep out of the fast path, if possible.
>>>>
>>>> I was thinking of keeping a function-pointer (in io_kiocb) during
>>>> submission. That would have avoided this check......but argument count
>>>> differs, so it did not add up.
>>>
>>> But that'd grow the io_kiocb just for this use case, which is arguably
>>> even worse. Unless you can keep it in the per-request private data,
>>> but there's no more room there for the regular read/write side.
>>>
>>>>>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
>>>>>> index 92c2269..2580d93 100644
>>>>>> --- a/include/uapi/linux/io_uring.h
>>>>>> +++ b/include/uapi/linux/io_uring.h
>>>>>> @@ -156,8 +156,13 @@ enum {
>>>>>>   */
>>>>>>  struct io_uring_cqe {
>>>>>>       __u64   user_data;      /* sqe->data submission passed back */
>>>>>> -     __s32   res;            /* result code for this event */
>>>>>> -     __u32   flags;
>>>>>> +     union {
>>>>>> +             struct {
>>>>>> +                     __s32   res;    /* result code for this event */
>>>>>> +                     __u32   flags;
>>>>>> +             };
>>>>>> +             __s64   res64;  /* appending offset for zone append */
>>>>>> +     };
>>>>>>  };
>>>>>
>>>>> Is this a compatible change, both for now but also going forward? You
>>>>> could randomly have IORING_CQE_F_BUFFER set, or any other future flags.
>>>>
>>>> Sorry, I didn't quite understand the concern. CQE_F_BUFFER is not
>>>> used/set for write currently, so it looked compatible at this point.
>>>
>>> Not worried about that, since we won't ever use that for writes. But it
>>> is a potential headache down the line for other flags, if they apply to
>>> normal writes.
>>>
>>>> Yes, no room for future flags for this operation.
>>>> Do you see any other way to enable this support in io-uring?
>>>
>>> Honestly I think the only viable option is as we discussed previously,
>>> pass in a pointer to a 64-bit type where we can copy the additional
>>> completion information to.
>>
>> TBH, I hate the idea of such overhead/latency at times when SSDs can
>> serve writes in less than 10ms. Any chance you measured how long does it
> 
> 10us? :-)

Hah, 10us indeed :)

> 
>> take to drag through task_work?
> 
> A 64-bit value copy is really not a lot of overhead... But yes, we'd
> need to push the completion through task_work at that point, as we can't
> do it from the completion side. That's not a lot of overhead, and most
> notably, it's overhead that only affects this particular type.
> 
> That's not a bad starting point, and something that can always be
> optimized later if need be. But I seriously doubt it'd be anything to
> worry about.

I probably need to look myself how it's really scheduled, but if you don't
mind, here is a quick question: if we do work_add(task) when the task is
running in the userspace, wouldn't the work execution wait until the next
syscall/allotted time ends up?

-- 
Pavel Begunkov

  reply index

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20200724155244epcas5p2902f57e36e490ee8772da19aa9408cdc@epcas5p2.samsung.com>
2020-07-24 15:49 ` [PATCH v4 0/6] zone-append support in io-uring and aio Kanchan Joshi
     [not found]   ` <CGME20200724155258epcas5p1a75b926950a18cd1e6c8e7a047e6c589@epcas5p1.samsung.com>
2020-07-24 15:49     ` [PATCH v4 1/6] fs: introduce FMODE_ZONE_APPEND and IOCB_ZONE_APPEND Kanchan Joshi
2020-07-24 16:34       ` Jens Axboe
2020-07-26 15:18       ` Christoph Hellwig
2020-07-28  1:49         ` Matthew Wilcox
2020-07-28  7:26           ` Christoph Hellwig
     [not found]   ` <CGME20200724155324epcas5p18e1d3b4402d1e4a8eca87d0b56a3fa9b@epcas5p1.samsung.com>
2020-07-24 15:49     ` [PATCH v4 2/6] fs: change ki_complete interface to support 64bit ret2 Kanchan Joshi
2020-07-26 15:18       ` Christoph Hellwig
     [not found]   ` <CGME20200724155329epcas5p345ba6bad0b8fe18056bb4bcd26c10019@epcas5p3.samsung.com>
2020-07-24 15:49     ` [PATCH v4 3/6] uio: return status with iov truncation Kanchan Joshi
     [not found]   ` <CGME20200724155341epcas5p15bfc55927f2abb60f19784270fe8e377@epcas5p1.samsung.com>
2020-07-24 15:49     ` [PATCH v4 4/6] block: add zone append handling for direct I/O path Kanchan Joshi
2020-07-26 15:19       ` Christoph Hellwig
     [not found]   ` <CGME20200724155346epcas5p2cfb383fe9904a45280c6145f4c13e1b4@epcas5p2.samsung.com>
2020-07-24 15:49     ` [PATCH v4 5/6] block: enable zone-append for iov_iter of bvec type Kanchan Joshi
2020-07-26 15:20       ` Christoph Hellwig
     [not found]   ` <CGME20200724155350epcas5p3b8f1d59eda7f8fbb38c828f692d42fd6@epcas5p3.samsung.com>
2020-07-24 15:49     ` [PATCH v4 6/6] io_uring: add support for zone-append Kanchan Joshi
2020-07-24 16:29       ` Jens Axboe
2020-07-27 19:16         ` Kanchan Joshi
2020-07-27 20:34           ` Jens Axboe
2020-07-30 16:08             ` Pavel Begunkov
2020-07-30 16:13               ` Jens Axboe
2020-07-30 16:26                 ` Pavel Begunkov [this message]
2020-07-30 17:16                   ` Jens Axboe
2020-07-30 17:38                     ` Pavel Begunkov
2020-07-30 17:51                       ` Kanchan Joshi
2020-07-30 17:54                         ` Jens Axboe
2020-07-30 18:25                           ` Kanchan Joshi
2020-07-31  6:42                             ` Damien Le Moal
2020-07-31  6:45                               ` hch
2020-07-31  6:59                                 ` Damien Le Moal
2020-07-31  7:58                                   ` Kanchan Joshi
2020-07-31  8:14                                     ` Damien Le Moal
2020-07-31  9:14                                       ` hch
2020-07-31  9:34                                         ` Damien Le Moal
2020-07-31  9:41                                           ` hch
2020-07-31 10:16                                             ` Damien Le Moal
2020-07-31 12:51                                               ` hch
2020-07-31 13:08                                                 ` hch
2020-07-31 15:07                                                   ` Kanchan Joshi
2020-08-05  7:35                                                 ` Damien Le Moal
2020-08-14  8:14                                                   ` hch
2020-08-14  8:27                                                     ` Damien Le Moal
2020-08-14 12:04                                                       ` hch
2020-08-14 12:20                                                         ` Damien Le Moal
2020-09-07  7:01                                                     ` Kanchan Joshi
2020-09-08 15:18                                                       ` hch
2020-07-31  9:38                                       ` Kanchan Joshi
2020-07-31  7:08                               ` Kanchan Joshi
2020-07-30 15:57       ` Pavel Begunkov

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